With the growing popularity of mobile terminals, mobile payments using mobile terminals has become in high and urgent demands. Although there are many implementation solutions, these solutions all have their disadvantages. Currently, methods adding radio frequency (RF) functions onto the subscriber identity module (SIM) of a mobile terminal (called an RF SIM card) or adding a short-distance communication module on the main board of a mobile terminal to achieve a short-distance communication of the mobile terminal have been developed. The later short-distance communication is called near field communication (NFC). These methods have enabled the mobile terminal to become a super smart terminal which can be used to recharge, shop, transact, or verify identification. This significantly fulfills the imperious market demands.
The short-distance solution based on the mobile-terminals with RF SIM cards has attracted extensive attentions because of its simplicity and no need to modify the mobile terminals. In such solution, the RF SIM card uses an ultra-high frequency (UHF) technology. Because the UHF SIM card, especially the SIM card using a 2.4 GHz⋅ISM public frequency range (i.e., the industry, science, and medical frequency), has a very high working frequency, and the size of antenna is small, RF signals with sufficient strength can be generated by putting a small antenna in a SIM card. Thus, even when the SIM card is installed inside a mobile terminal, the RF signals can still emit from the mobile terminal. An industry-widely-used RF transceiver chip installed in a card reader can reliably receive RF signals from a majority of the mobile terminals without extra amplification. Thus, this can enable the mobile terminals to achieve the short-distance communication function without modifying the existing mobile terminals. However, current technologies often fail to effectively control the communication distance between the RF card and a card reader.